408 METALLURGY OF IRON" AND STEEL. In addition to this effect, I believe an equally important factor exists in 'the action of carbon, phosphorus, sulphur and copper in destroying the cohesion by increasing the tendency to crystallization, for these metalloids lower the point at which the steel becomes what is incorrectly, but quite naturally, called "burned/3 When steel is overheated it crumbles under the hammer, and it cannot be easily united to another piece when it is incapable of remaining united to itself. This theory also explains what seems to be a fact, that a small proportion of manganese aids in welding, for although it does decrease the mobility at any particular temperature, it allows a higher heat to be put upon the metal without destructive crystallization, and thus indirectly renders possible a greater mobility and maintains a more favorable molecular structure. The following conclusions seem to fit the theory and the facts: '(1) With the exception of manganese in small proportion, the usual impurities in steel reduce its welding power by lowering the critical temperature at which it becomes coarsely crystalline. (2) A small content of manganese aids welding by preventing crystallization. (3) Only the purest and softest steel can be welded with any reasonable assurance of success. (4) The confidence of a smith in his own powers and in the perfection of the weld is no guarantee that the bar is fit to use.